change the default to PyMYSQL

As discussed in the Liberty Design Summit "Moving apps to Python 3"
cross-project workshop, the way forward in the near future is to
switch to the pure-python PyMySQL library as a default.

    https://etherpad.openstack.org/p/liberty-cross-project-python3

Change-Id: Ic609ce136061b753ca692b37509a0b29c60bb8b5
diff --git a/lib/databases/mysql b/lib/databases/mysql
index 7cd2856..832c2ca 100644
--- a/lib/databases/mysql
+++ b/lib/databases/mysql
@@ -11,7 +11,7 @@
 MY_XTRACE=$(set +o | grep xtrace)
 set +o xtrace
 
-MYSQL_DRIVER=${MYSQL_DRIVER:-MySQL-python}
+MYSQL_DRIVER=${MYSQL_DRIVER:-PyMySQL}
 # Force over to pymysql driver by default if we are using it.
 if is_service_enabled mysql; then
     if [[ "$MYSQL_DRIVER" == "PyMySQL" ]]; then